An imaging radar transmits electromagnetic waves to objects within a region of interest that scatter, or reflect, the energy according to the properties of such objects (e.g. physical attributes). When the radar receives reflected waves from the objects a spatial distribution of the reflected energy may be constructed by a processor. This distribution defines a radar image that may be displayed as a 3 dimensional representation of the scene or any 2-dimensional representation thereof.
A real-aperture imaging radar, as opposed to a synthetic-aperture imaging radar (SAR), irradiates each location in the scene in turn. The line-of-sight (range) distance to a location is determined by the round-trip time of the reflected pulse; resolution in the range direction is determined by the duration of the reflected pulse; the cross-range location is determined by the pointing angles of the antenna; the cross-range resolution is determined by the beam width of the antenna. The range resolution can be less than a meter but conventional microwave antenna beamwidths may limit the resolution in the angle coordinate (i.e. cross range limitation).
This limitation may be overcome by synthesizing the effect of a large antenna. That is, the radar may be located on a moving vehicle and the reflected waves of successive radar pulses may be measured and stored in a memory. In this configuration the actual beam is wide enough to illuminate the entire scene at once, and does not have to scan from one location to the next in order to form an image. The stored reflected wave information is coherently processed to obtain a synthetic aperture radar (SAR) image.
In use, SARs may function as all-weather imaging radar systems and typically produce two-dimensional (2-D) or three-dimensional (3-D) images, with intensity as an additional non-spatial image dimension. Data collection and data processing may be performed by a variety of methods. Regardless of method, the location of a stationary point in the scene is determined by the temporal profile of both line-of-sight distance between the point and the collection antenna (however, the information is generally not used directly in this form). The processor attempts to make sense of the collected data by assuming that all points in the associated scene are stationary.
Conversely, a rigid object may be imaged if the collector is stationary and the object moves past the collector. This method is called inverse SAR (ISAR). Similar to the SAR, ISAR also produces two-dimensional or three-dimensional images with intensity as an additional non-spatial image dimension. However, ISARs use the motion of a viewed object to synthesize a large aperture antenna and not the motion of the radar platform itself. Both the translational and rotational motion of the object must be accurately known. Features of SAR systems may be equally applicable to ISAR and, where applicable, the two systems may be referred-to generally as being SAR systems and may be configured separately or in combination.
While conventional systems have been quite successful in presenting images to trained personnel for interpretation, drawbacks have been encountered. Specifically, moving objects may not be visible in traditional SAR imagery due to the fact that radar energy may be widely scattered in the image by the processor. Conventional methods have used special radar systems that are designed to see and/or track moving objects, but which cannot form, using either an integrated process or a separate process, an image of the object's environment. Since such mover detection methods are a part of the image formation process and cannot be readily separated therefrom, special tasking, special collections, or special image processing are conventionally required for detecting moving objects using SAR imaging.